An image sensor device is an integrated circuit (IC) having an array of pixels and other circuitry and devices for sampling the pixels, outputting the sample values and processing the sample values. One problem that arises in image sensor ICs is that switching events that occur on the IC may lead to noise on the power supply. The pixels are typically sampled one row at a time during sampling periods. If a switching event or other noisy event occurs while a row of pixels is being sampled, this may lead to noise in the sample signal. This of course is problematic.
Most Complementary Metal Oxide Semiconductor (CMOS) image sensors that implement 4T buried-gate pixel technologies use correlated-double sampling to remove power supply noise present in the reset value of the pixel. With this technique, reset and video voltage values that are independent of the power supply are provided to the analog read-out line. These values must then be sampled and stored on analog storage devices (usually capacitors) that are accessed at a later time for analog-to-digital conversion of the samples and image processing. Because there is only one external route from the pixel to the storage element, the sampling of the reset and video voltage values must occur at two separate instants in time. This provides an opportunity for power supply noise to be added to the sample signal.
The techniques that are used to address this problem are directed to minimizing power supply noise. With one of the techniques, the digital circuits are removed from the image sensor IC and placed in a separate IC. This requires that a signal be driven between the image sensor IC and the digital circuit IC, which involves driving the signal over routes on the circuit board, which increases capacitance and power consumption. In addition, this solution is often cost prohibitive.
Another technique for minimizing power supply noise actively drives the analog power supply low and high to eliminate power supply noise. One of the disadvantages of this technique is that it requires a large amount of power in order to be effective. A special amplifier is needed and the amplifier must be capable of supplying the full power supply current to the power supply. In addition, it also requires a second, wider power supply to provide power for the amplifier.
Another technique for minimizing power supply noise uses separate power and ground nets for all digital and analog circuitry. This is a very complicated power system with multiple regulators, which can also be cost prohibitive. In addition, this technique does not remove substrate noise due to the fact that all of the circuits on the IC share the same ground, i.e., the substrate.
A need exists for a way to prevent power supply noise from affecting the pixel sample signals that is relatively simple to implement, and which is efficient in terms power consumption and area.